The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Many motor vehicle powertrain components have been and are fabricated of steel because of its strength, malleability, cost and availability. This is not to say, however, that all its engineering attributes are positive or desirable. One of its most commonly recognized drawbacks is its weight, especially when compared to materials such as aluminum and magnesium.
A conventional motor vehicle automatic transmission may include one or more friction clutch assemblies, planetary gear assemblies, band brakes, accumulators, spline sets, valve bodies and wear surfaces engaged by bushings, seals, springs and similar components. A conventional friction clutch includes a friction clutch pack having alternating friction plates and reaction plates, both typically fabricated of steel, disposed between an apply plate and a backing plate against which the friction clutch pack is compressed. When compressed, torque is transferred through the friction clutch pack between an inner hub and an outer housing. Band brakes comprehend a circumferential band having friction material which partially encircles a rotating cylindrical body which may be a housing or similar component. One end of the brake band is fixed and the other end is coupled to an actuator which tightens the band about the cylindrical body.
Between the clutch hub or housing and other components such as shafts, quills or planetary gear components, it is often necessary to establish a fluid tight seal in order to either separate fluid filled, i.e., wet, areas from dry areas or to establish a flow path for fluid across or between rotating and non-rotating components. This may be achieved by, in the first case, a resilient seal disposed in a channel or groove between the areas or, in the second case, utilizing a pair of resilient seals disposed in grooves on both sides of the flow path to properly direct fluid flow. Also, one or both of the inner hub and outer housing may be coupled to a shaft or quill by splines.
Conventionally, friction clutch plates, band brake cylinders, hubs, housings, spline sets and components having surfaces engaged by friction material or resilient seals have been fabricated of various steels due to the strength and durability these materials. Not only does steel provide the structural integrity necessary to carry the torque loads encountered by such components but it also provides the durability of the faces or surfaces exposed to frictional (sliding) forces. As noted, the primary drawback of such steel clutch components is weight. However, since many of these transmission components both contribute to the total vehicle weight and rotate and thus contribute to powertrain energy loss as they are repeatedly accelerated and decelerated, they twice contribute to fuel consumption. Accordingly, any weight reduction of a rotating component, such as a clutch or other transmission component, positively affects fuel consumption, both from the standpoint of total vehicle weight as well as powertrain efficiency.
One solution to the weight problem is the substitution of a lighter weight material such as aluminum or magnesium. Aluminum has about one-third the density of steel and exhibits good heat transfer, both characteristics being important in transmissions, especially clutches and brakes. However, it has been found that this material is unsuitable because the surfaces exposed to frictional forces during clutch and brake engagement as well as those exposed to sliding forces such as seals and valve surfaces exhibit unacceptable wear and thus unacceptable service life.